1. Field of the Invention
This invention relates to an automotive navigation system, and more particularly to an automotive navigation system having a satellite-utilizing positioning system which receives radio waves from artificial satellites and detects the present position of a vehicle.
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publication No. 58(1983)-70117, there has been known an automotive navigation system in which the present position of the vehicle is shown on a display device together with a map of the area around the present position. As the means for detecting the present position of the vehicle, there has been put into practice a present position detecting system in which an azimuth sensor such as a geomagnetism sensor is employed. That is, the traveling distance and the traveling azimuth of the vehicle from a reference position are detected by means of a vehicle speed sensor and an azimuth sensor, and the present position of the vehicle is determined on the basis of the detected traveling distance and the detected traveling azimuth. However, such a present position detecting system is disadvantageous in that since the present position is measured as, say, a position relative to the reference position, the accuracy of the positioning is deteriorated due to measuring errors in the traveling distance and/or the traveling azimuth.
Such a problem can be overcome by measuring the present position of the vehicle as, say, an absolute position by use of radio waves transmitted from artificial satellites. For example, this can be accomplished by use of a GPS (Global Positioning System) which is now in the process of development. By use of a GPS, the present position of a vehicle can be determined with an accuracy of measurement of about 30 meters (in the case of C/A code which is to be opened to the public) on the basis of radio waves transmitted from four artificial satellites (generally called "NAVSTAR").
In the vehicle provided with a navigation system having a satellite-utilizing positioning system in which the present position of the vehicle is detected by use of a GPS, a map of the area where the vehicle is traveling is displayed, for instance, as a picture on a CRT carried by the vehicle and the detected present position is shown as a spot on the picture. The present position must be displayed in response to movement of the vehicle, and accordingly, the radio waves from the respective satellites are repeatedly received by the navigation system at predetermined intervals, and the present position is determined each time the radio waves are received.
In such a navigation system, while the vehicle is at a stop, the spot representing the present position must remain stationary on the picture. However, since the detected present position for a given position of the vehicle can fluctuate within about 30 meters as described above and the fluctuation is apt to be increased depending on the positions of the satellites and/or radio interference, the spot representing the present position of the vehicle moves, due to the fluctuation in the detected present position, each time the radio waves from the satellites are received and the present position of the vehicle is determined on the basis of the received radio waves, though actually the vehicle remains stationary.